The present invention relates to a shift control method for a sub-transmission incorporated into the power transmission system of a vehicle such as an automobile, and more particularly relates to a shift control method for performing properly the shifting of such a sub-transmission in accordance with the operating conditions of the vehicle, bearing in mind the style of driving and of gear shifting of the driver.
It is known to incorporate a two-speed sub-transmission, which can be shifted between a high speed stage and a low speed stage, into the power train of a vehicle such as an automobile which also is equipped with a manual transmission.
The control of such a sub-transmission has sometimes been manual; however, this puts a strain on the driver, and distracts him unduly from the business of vehicle control. As a result, such manual control has not been practical from the point of view of obtaining the proper shift timing of the sub-transmission to ensure good performance and fuel economy.
In order to avoid this problem, it has been proposed to control such a sub-transmission electrically or electronically, by shifting it over from the low speed stage to the high speed stage, and from the high speed stage to the low speed stage, automatically, depending upon combinations of various vehicle and/or engine operating parameters, such as the vehicle speed, the engine revolution speed, the throttle opening, the engine load, etc.. Typically, the shifting is performed in accordance with a transmission diagram, which is a line on a graph of a signal representing either vehicle speed or engine speed, against a signal representing engine load, such as the throttle opening.
Now, the shifting of the manual transmission is performed by the driver, according to his own free will, and the precise shift points will vary according to the driver's habits, the conditions of the road, and so on. If, therefore, the transmission diagram of the sub-transmission is determined independently of these conditions, and shifting of the sub-transmission is performed in accordance therewith, it may occur that this shifting is performed at times which rather go against the wish of the driver, as expressed by his particular current pattern of gear shifting.
For instance, if the driver currently does not perform upshifting until the speed of the vehicle has become relatively high, so that he is performing so-called high-revving or high acceleration operation, it is desirable that the sub-transmission, responding to this mode of operation, likewise should not shift from its low speed stage to its high speed stage until the vehicle speed has become relatively high. Thus the driving style of the driver and the sub-transmission will blend harmoniously.
If, on the other hand, the sub-transmission does not respond to this high-revving mode of operation by the driver, then the shift of the sub-transmission from its low speed stage to its high speed stage may occur almost immediately after the manual upshift by the driver, so that, in effect, high-revving operation is not performed, and the driver's intentions are frustrated.
Similarly if, on the other hand, the driver is currently in the habit of performing upshifting rather early, so that the vehicle is being operated in a low-revving or sedate mode, (which provides good fuel economy), then, again, if the automatic control of the sub-transmission does not respond to this mode of manual shifting, a rather long time will elapse after upshifting by the driver before the sub-transmission shifts from its low speed stage to its high speed stage, and thus low-revving operation of the vehicle will not take place, and again the driver's intentions will be frustrated.